I. Field of the Invention
This invention relates generally to apparatus for effectively eliminating electrostatic discharge problems in laboratory and/or manufacturing facilities, and more particularly to a room air ionization system wherein aesthetic and maintenance problems resulting from unsightly dirt buildup is significantly reduced.
II. Discussion of the Prior Art
Electrostatic discharge (ESD) is of considerable concern to electronic systems manufacturers, especially where semiconductor devices, including microcircuits, are being manufactured, handled or assembled onto printed circuit cards. As the complexity of microcircuits increases and the spacing between the microscopically fine lines and active elements decrease, the propensity to damage caused by static electricity discharges significantly increases. Failure of one or more discrete elements within an integrated circuit package may render an entire electronic assembly inoperative, requiring replacement and costly rework.
In combating ESD problems, electronics manufacturers have developed various approaches for reducing the buildup of potentially damaging electrostatic voltages on objects, such as tools, test equipment and persons working with those devices. For example, raising the relative humidity in the work area to approximately 40% or more causes a decay of electrostatic charge, but this can and often does lead to worker discomfort as well as a relatively long charge decay time. A high humidity environment also leads to various other manufacturing problems and can impact product reliability adversely. Personal grounding wrist straps, grounding pads, conductive floors and anti-static fabrics may be used in an effort to reduce the ESD potentials below the sensitivity rating of the electronic components being handled. The above approaches are of varying effectiveness and have their own drawbacks unique to the particular approaches employed.
In the Mykkanen et al U.S. Pat. No. 4,476,514, which is assigned to the assignee of the present invention, there is described a system for maintaining a degree of ionization in the atmosphere of work spaces which will result in neutralization of static electric charges. Spaced pairs of insulated conductors are suspended proximate the ceiling of the work area and are maintained at constant high positive and negative potentials with respect to ground. At regularly spaced locations along the lengths of the positive and negative bus bars are ion-producing electrodes in the form of sharply pointed pins which penetrate the insulation and contact the conductive portion of the bus bars. The voltage applied to the bus bars is sufficiently high to create a corona discharge at the electrode points.
The system described effectively obviates electrostatic discharge problems in the work environment by flooding the area with essentially equal numbers of positive and negative ions. These ions are electrically attracted to electrostatic charges of opposite polarity existing in the work area and neutralize such charges to the point where voltage breakdowns which could potentially damage sensitive electronic components are eliminated. The charge decay time using the ionization scheme is much more rapid than when 40% relative humidity is used.
The system described in the aforereferenced patent, employs dual emitter lines which are continuously energized by a constant amplitude DC voltage, one line being energized with a positive potential relative to ground and the other with a negative potential relative to ground. Thus, the high voltage current effectively flows in one direction through the individual emitter lines. This has been found to cause the positive emitter bars and surrounding objects in close proximity to the dual emitter lines to become dirty within a fairly short time. When it is considered that considerable time and money may be required to maintain the facility in an aesthetically pleasing condition, any way to reduce the frequency of the necessary cleaning operation constitutes an overall benefit to the company in question.
It is also known that the effectiveness of room ionization system in preventing ESD has a considerable dependence upon the spacing or distance between the item bearing the electrostatic charge and the ion generation grid structure. It is found that at points close to an emitter pin, discharge of an electrostatically charged sample of the same polarity as the ions being emitted from the emitter needle in question is significantly slower than that which is observed at a somewhat greater separation between the sample and the emitter needle. This is due to the fact that at locations very close to the emitter pin, opposite polarity ions have had little time in which to mix. In addition, the ion wind from the emitter needles does have some force near those needles and can, within limits, overcome the repulsion of a like charged object and actually add additional charge to it. This is referred to as "proximity charging" and is undesirable.